First-principles study of ethylene on Ge(001)-electronic structures and STM images

By using the first-principles density functional theory, we calculate the partial charge densities and STM images for the intradimer di-σ and interdimer end-bridge adsorption configurations of ethylene on Ge(001). Our simulated STM images show the effects of ethylene adsorption and clarify that, although STM images and surface structures evolve as the adsorption sites of ethylene on Ge(001), the molecular orbitals of the bare Ge atoms always remain the dominating electronic states near the Fermi level. For the di-σ model, the display of such dominance in STM images is, however, damped by the preferred tunneling paths between the tip and the electronic states of ethylene due to their short tunneling distances. In comparison, the distance between the tip and the end-bridge bound C₂H₄ molecules is not so short relative to the distance between the tip and the up-Ge atoms of the bare Ge-Ge dimer; hence, the dominance of bare up-Ge atoms can still be found in the STM images at low bias voltages. Our simulated STM results confirm that the di-σ and pairedend- bridge configurations are observable adsorption structures for C₂H₄ on Ge(001). The comparisons of the STM images between Ge(001) and Si(001) reveal the distinction of their highest occupied surface states, which explains the differences in geometry and reactivity of adsorbates, including C₂H₄ and O ₂, on Ge(001) versus Si(001).